The present invention relates to an electronic charger for electric power distribution installations operating at medium voltage level.
For the purposes of the present invention, the term medium voltage (MV) identifies operating voltages higher than 1 kV AC and 1.5 kV DC up to some tens of kV, e.g. up to 72 kV AC and 100 kV DC.
Electric power distribution installations operating at MV level may comprise, for example, MV switchboards, MV power supply systems, MV switching apparatuses (e.g. circuit breakers, contactors, disconnectors, reclosers, etc.) and the like.
As is known, in many installations of the type above capacitor banks or batteries are adapted to store huge amounts of electrical energy (typically some thousands of Joule).
This is particularly true for MV switching apparatuses comprising an electromagnetic actuator. In these devices, the power supply stage is not generally designed to harvest the electric power needed for operating the actuator directly from the available power supply.
Such a choice would in fact entail relevant drawbacks in terms of size, industrial costs and component assembling since the actuator may require high peaks of electric power during operation.
For this reason, the power supply stage of a MV switching apparatus generally comprises one or more capacitor banks, which are adapted to store electrical energy to properly feed the actuator even when relatively high electric power peaks are needed for operation.
In electric power distribution installations operating at MV level, an electronic charger is typically provided to charge such huge capacitive loads by drawing electric power from the available power supply (e.g. a MV electric line).
Generally, a traditional electronic charger comprises a DC/DC switching converter of fly-back type or boost type.
Unfortunately, the experience has shown that charging such huge capacitive loads is a difficult process to manage.
The adoption of linear charging techniques is not suitable as the electronic charger should be designed to provide a DC voltage higher than the final charging voltage desired for the capacitor banks. This would entail relevant drawbacks in terms of electric power dissipation.
On the other hand, due to their huge size, these capacitive loads electrically behave as short circuits at the beginning of the charging process and as open circuits, when the charging process is almost complete and the final charging voltage is almost reached.
Thus, ideally, current control techniques should be adopted at the first steps of charging process whereas voltage control techniques should be adopted, when the voltage across the capacitor banks is closer to the desired final charging voltage.
Traditional electronic chargers are still not able to provide performances of this kind.
Further, they show some additional drawbacks.
Electronic chargers, which comprise a DC/DC fly-back converter, typically show high electric power dissipation as leakage currents, which are due to the leakage inductance of the converter transformer) are present when the converter switch is in an OFF state (interdiction or cut-off state).
In addition, electronic chargers of this type generally offer poor performances in terms of output voltage regulation as the winding ratio of the converter transformer substantially determine whether the converter is of the step-up type or step-down type.
Further, these electronic chargers show high reflected voltages on the converter switch, when this latter switches in an OFF state. Such reflected voltages may cause stress failures on the converter switch.
Electronic chargers, which comprise a DC/DC boost converter, typically show high current ripples at the input stage. These phenomena may entail high electromagnetic emissions and shortening of the useful life of the electronic components. Expensive circuit configurations need often to be arranged to deal with these problems.